Alkylene glycols, in particular monoalkylene glycols, are of established commercial interest. For example, monoalkylene glycols are being used in anti-freeze compositions, as solvents and as base materials in the production of polyalkylene terephthalates e.g. for fibres or bottles.
The production of alkylene glycols by liquid phase hydrolysis of alkylene oxide is known. The hydrolysis is performed without a catalyst by adding a large excess of water, e.g. 20 to 25 moles of water per mole of alkylene oxide, or it is performed with a smaller excess of water in a catalytic system. The reaction is considered to be a nucleophilic substitution reaction, whereby opening of the alkylene oxide ring occurs, water acting as the nucleophile. Because the primarily formed monoalkylene glycol also acts as a nucleophile, as a rule a mixture of monoalkylene glycol, dialkylene glycol and higher alkylene glycols is formed. In order to increase the selectivity to monoalkylene glycol, it is necessary to suppress the secondary reaction between the primary product and the alkylene oxide, which competes with the hydrolysis of the alkylene oxide.
One effective means for suppressing the secondary reaction is to increase the relative amount of water present in the reaction mixture. Although this measure improves the selectivity towards the production of the monoalkylene glycol, it creates a problem in that large amounts of water have to be removed for recovering the product.
Considerable efforts have been made to find an alternative for increasing the reaction selectivity without having to use a large excess of water. Usually these efforts have focused on the selection of more active hydrolysis catalysts and various catalysts have been disclosed.
Both acid and alkaline hydrolysis catalysts have been investigated, whereby it would appear that the use of acid catalysts enhances the reaction rate without significantly affecting the selectivity, whereas by using alkaline catalysts generally lower selectivities with respect to the monoalkylene glycol are obtained.
Certain anions, e.g. bicarbonate (hydrogen carbonate), bisulphite (hydrogen sulphite), formate and molybdate, are known to exhibit good catalytic activity in terms of alkylene oxide conversion and selectivity towards monoalkylene glycol. However when the salts of these anions are used as the catalyst in a homogeneous system, work-up of the reaction product by distillation will pose a problem because the salts are poorly soluble in the glycol and tend to make it semi-solid. Quaternary ammonium salts remain soluble in the glycol reaction product.
High conversions, good selectivity and a low water/alkylene oxide ratio can be obtained with the process, disclosed in EP-A 0 156 449 and EP-A 0 160 330 (both of Union Carbide). According to these documents the hydrolysis of alkylene oxides is carried out in the presence of a selectivity-enhancing metalate anion-containing material, preferably a solid having electropositive complexing sites having affinity for the metalate anions. The said solid is preferably an anion exchange resin, in particular a styrene-divinyl benzene copolymer. The electropositive complexing sites are in particular quaternary ammonium, protonated tertiary amine or quaternary phosphonium. The metalate anions are specified as molybdate, tungstate, metavanadate, hydrogen pyrovanadate and pyrovanadate anions. A complication of this process is that the alkylene glycol-containing product stream also comprises a substantial amount of metalate anions, displaced from the electropositive complexing sites of the solid metalate anion containing material. In order to reduce the amount of metalate anions in the alkylene glycol product stream, this stream is contacted with a solid having electropositive complexing sites associated with anions which are replaceable by the said metalate anions.
In WO 95/20559 (Shell) there is disclosed a process for the preparation of alkylene glycols wherein an alkylene oxide is reacted with water in the presence of a catalyst composition comprising a solid material having one or more electropositive sites, which are coordinated with one or more anions other than metalate or halogen anions, e.g. bicarbonate, bisulphite and carboxylate, with the proviso that when the solid material is an anionic exchange resin of the quaternary ammonium type and the anion is bicarbonate the process is performed in the substantial absence of carbon dioxide. According to this document, the presence of carbon dioxide in the feed is detrimental to the catalytic effect of bicarbonate-exchanged resins of the quaternary ammonium type.
A drawback shared by the conventional anionic exchange resins is their limited tolerance to heat. In practising the process of alkylene oxide hydrolysis according to WO 95/20559 with catalyst compositions based on conventional organic quaternary ammonium ion exchangers it has been found, that under severe alkylene oxide hydrolysis reaction conditions (high temperature and/or long service) the catalytic activity (selectivity and/or conversion) of the conventional resin-based catalysts tends to deteriorate. Moreover, under these reaction conditions these catalysts were found to undergo swelling.
The sensitivity to heat of anionic exchange resins has been known for a long time. According to Elizabeth W. Baumann, in J. of Chemical and Engineering Data 5 (1960) 376-382, the degradation of AMBERLITE IRA-400 which is a strong base (quaternary ammonium) ion exchange resin having three methyl groups in its quaternary structure can (according to two decomposition reactions 1a and 1b) liberate trimethylamine which can be absorbed by a cation exchange resin such as AMBERLITE IR-120-H, if present, or methanol which is not absorbed by the cation exchange resin. In the first column of the article it is further remarked, that "the presence of this resin [AMBERLITE IR-120-H] provides a means for absorbing basic decomposition products that might affect the progress of decomposition, permits study of the decomposition by reaction 1a and roughly duplicates the conditions in a mixed bed deionization system". The article contains no showing of any effect of AMBERLITE IR-120-H, which is a strongly acidic ion exchange resin of the sulphonic type, on the thermal stability of the anionic exchange resin. And of course the article is not concerned with the stability of any catalytic effect associated with an ionic exchange resin.
In U.S. Pat. No. 4,579,983 (Union Carbide) there is disclosed a process for making alkylene glycols from alkylene oxide and water in the presence of a water-insoluble phase containing a selectivity-enhancing organometalate which may comprise an anion exchange resin, and a stabilising material which is water-soluble and comprises a cation and a selectivity-enhancing metalate anion. This stabilising material is thus a metalate salt.
In the co-pending European Patent Application No. ........, filed on even day herewith, there is disclosed a process for the preparation of alkylene glycols by reacting an alkylene oxide with water in the presence of a catalyst composition including a carboxylic acid derivative, having in its chain molecule one or more carboxyl groups and one or more carboxylate groups, the individual carboxyl and/or carboxylate groups being separated from each other in the chain molecule by a separating group consisting of at least one atom. Catalysts compositions including such carboxylic acid derivatives immobilised on a solid support, in particular an anionic exchange resin, are specifically claimed. One advantage of the carboxylic acid derivatives as defined in this application is that their catalytic combination with anionic exchange resins is more stable.
It has now been found that the stability of solid catalysts in the conversion of alkylene oxide to alkylene glycol, which solid catalysts include a strongly basic ion exchange resin coordinated with one or more anions, can be considerably enhanced by adding a relatively small amount of an acidic ion exchange resin.